1. Field of the Invention
The invention relates to the removal of contaminants from streams of ammonia gas and liquid. More particularly it relates to the production of substantially contaminant-free streams of ammonia gas or liquid for use in the production of semiconductors and similar products which cannot tolerate the presence of such contaminants during manufacture.
2. Background Art
There is current research to develop high performance light emitting diodes (LEDs). Such LEDs are intended for such disparate uses as in outdoor displays, vehicles, traffic signals, lasers, medical devices and indoor lighting, and are expected to replace the current bulbs or fluorescent lighting tubes. The high performance LEDs emit light of specific wavelengths compared to broad spectrum wavelength emissions of prior bulbs and tubes, so that optimal light spectra can be provided for each end use by combination of LEDs of different colors. For instance, LED combinations with spectra matching incandescent lighting (as compared to the spectra of flourescent lighting) can be provided for indoor use, without creating the heat generation and dispersion problems caused by incandescent bulbs.
These LEDs are made by metal organic chemical vapor deposition (MOCVD). using materials such as gallium-, aluminum gallium- and indium gallium nitrides and phosphides. In a process currently of significant interest, gallium nitride is deposited from a gaseous mixture of ammonia, hydrogen and trimethyl gallium. Similar, gallium nitride is being considered for xe2x80x9cblue lasers,xe2x80x9d i.e. lasers which emit blue light. Because blue light has a shorter wavelength than red, yellow or green light, blue lasers are anticipated to be capable of forming compact disks which will have a much higher information density than is presently the case with compact disks produced with red laser light. Gallium nitride for such blue lasers would be manufactured in the same type of ammonia/hydrogen/trimethyl gallium gaseous environment as described above for the high capacity LEDs.
The LEDs, blue lasers and integrated circuits are all manufactured with electron accepting p-type dopants. Such products are extremely sensitive to the presence of electron-donating n-type materials, and very small concentrations of such n-type are sufficient to deactivate the p-type dopants and impair or destroy the performance and operability of the integrated circuits, LEDs and blue lasers. Oxygen is a particularly efficient n-type material, and the presence of molecular oxygen causes lattice defects and is detrimental to the desired band gap properties in the semiconductor or laser material. Even very low concentrations of oxygen ( less than 100 ppb, usually  less than 50 ppb, even  less than 10 ppb) can be sufficient to cause sufficient reduction in performance or operability (especially in wavelength control) so as to require discarding of the product after manufacture or to significantly shorten operating lifetime. Similar detrimental effects are observed with similar low concentrations of water or hydrocarbons in the manufacturing system.
There are also numerous manufacturing processes of current interest in which ammonia is a major component in the production of high purity products. Commonly these processes use ammonia in gaseous form, but liquid ammonia is also used to some extent. In addition, a liquid ammonia component frequently is vaporized during a process to be used subsequently in gaseous form. A common requirement in these processes is that all reactants, catalysts, carriers, etc. must have the least practical contaminant level, since the products produced must be of very high purity. Examples include the following.
Recent advances in integrated circuit semiconductor technology have included the development of semiconductors with copper interconnects instead of aluminum interconnects. Copper interconnects are advantageous in that copper has less resistance than aluminum, which leads to higher performance in microprocessors, microcontrollers and random access memories. However, copper tends to migrate over a period of time, so it is necessary to construct barrier layers in the semiconductor to prevent the copper migration. Such boundary layers are typically made of nitrides such as tantalum nitride, titanium nitride or silicon nitride. These layers are commonly formed by deposition from a hydride gas, e.g., ammonia.
Ammonia is widely used as a source of nitrogen for film development in some thin film applications. The ammonia allows for lower temperature film growth in chemical vapor deposition (CVD) processes.
As mentioned, in addition to oxygen contamination, the presence of water vapor, gaseous hydrocarbons and/or carbon dioxide gas in hydride gases such as ammonia is also detrimental, since those materials lead to degradation of the products formed by deposition of active layers of metals or metal compounds from a hydride gas environment. Water is one of the most common and yet most difficult impurities to remove from the gases. Water is of course ubiquitous in almost all ambient environments. Even systems which are nominally referred to as xe2x80x9cdryxe2x80x9d usually have significant amounts of water, and most drying processes can reduce the moisture content of a gas only to a xe2x80x9cminimumxe2x80x9d which is still in the parts per million (ppm) range. However, since for many purposes water contents in the ppm range are quite acceptable, there are numerous patents and articles in the literature dealing with such types of xe2x80x9cppm drying processes.xe2x80x9d
In the manufacture of such products, moisture contents of the depositing gases which are in the ppm range are excessively wet. To form satisfactory products, the water content of the depositing gases must be reduced to the parts per billion (ppb) range, usually down to no more than about 100 ppb. See Whitlock et al, xe2x80x9cHigh Purity Gases,xe2x80x9d in Ruthven, ed., Encyclopedia of Separation Technology, vol. 1, pp. 987-1000 (1997).
Attempts to use materials such as reduced nickel or copper catalysts to remove contaminants such as oxygen, carbon dioxide and water from hydride gases have not been successful. While contaminant removal can be effected for short periods of time down to the 10 ppb level, the reactive effects of the hydride gases, especially ammonia, very quickly cause the materials to degrade and contaminate the gas stream with metal complexes. Though pre-existing impurities may be reduced, the introduction of new impurities to the manufacturing process is unacceptable.
Processes have been described in which oxygen has been removed from ammonia streams by metals serving as xe2x80x9cgetters.xe2x80x9d However, these have been relatively ineffective at reaching sufficiently low levels of decontamination. In addition, the getters are deposited on substrates, such as silica or zeolites, which do not play a central role in the decontamination process, and also may themselves be degraded by the hydride gases. See, for instance, U.S. Pat. No. 5,496,778 (Hoffman et al.), U.S. Pat. No. 5,716,588 (Vergani et al.) and U.S. Pat. No. 4,976,944 (Pacaud et al.); PCT publication No. WO 97/06104 (SAES Getters S.p.A.); and European Patent No. EP 0 784 595 B1 (SAES Getters S.p.A.). In particular, some of these references teach that manganese:iron ratios of  greater than 2:1 as depositions on such substrates are detrimental to getter performance and are to be avoided. The references specifically teach that very low manganese:iron ratios, usually about 0.012-0.16:1, are to be preferred. Further, the reference processes are usually not effective for removal of carbon dioxide or water, as compared to oxygen, from ammonia gas streams.
Consequently, the problem of removal of contaminant levels down to xe2x89xa6100 ppb from ammonia remains a significant problem in the field of production of high purity LEDs, blue lasers, semiconductors, and the like. Those processes which are being used are expensive because of the very short service life of the decontaminating materials and the need for their frequent replacement. In addition, since it is difficult to determine the exact rate of deterioration of the decontaminating materials in the presence of the ammonia, users of such decontaminating materials must schedule their discard and replacement at intervals less than the shortest expected service life. To do otherwise would risk failure of a decontamination unit with the resultant loss of contaminated product when the excessive contaminant concentrations reaches the production chamber through the failed unit. Consequently, the current systems require that many if not most of the decontamination units must be discarded while they still have some degree of useful service life left, thus further increasing the expense of the system operations. This must be considered against the background that the market expects that manufacturing processes must have a continuous pure gas flow at high flow rate with consistent levels of purification and low cost of ownership. Therefore the more that a process deviates from these anticipated parameters, the less acceptable it is. The successful processes are those which have sufficiently high levels of purification to permit the maximum practical operating lives at reasonable cost.
While ammonia decontamination systems have been described that include multiple adsorbent beds or vessels that alternate in decontaminating and regenerative function, such systems suffer drawbacks in that they either a) require the periodic replacement of adsorbent, e.g., a getter alloy, and/or b) involve a regenerative process that relies on the administration of gas from outside the system to generate fresh adsorbent, e.g., alumina, from contaminant-saturated adsorbent. See, e.g., U.S. Pat. No. 5,833,738 (Carrea et al.).
Until the present invention, the above processes were limited by the need to add exogenous supplies of gas, for instance from cylinders, to the system. This required more handling and plumbing, which in turn exacted a toll on overall process efficiency.
If the supply of regenerant gas or fresh adsorbent could be supplied from within the system itself, it would simplify and enhance existing purification and gas delivery systems, thereby resulting in lower operating costs, which savings in turn could be passed on to the benefit of the consumer.
The invention herein is of an improved ammonia decontamination system which includes the capability of self-regeneration of the metal or metal oxide decontaminant/adsorbent. Regeneration of spent or contaminated adsorbent (highly oxidized) is accomplished by reducing the metal adsorbent to lesser oxidized (more reduced) states using hydrogen gas which is produced by cracking a fraction of the purified ammonia product, such that there is no need to supply large amounts of hydrogen from external sources. When there are at least two ammonia purifying reactors in parallel and at least one ammonia cracking reactor, the process is both self-contained and capable of continuous operation. This translates to reduced down time, less servicing and handling, increased safety and increased efficiency.
The process of the invention can be operated for long periods of time owing to the efficient self-regeneration of adsorbent using controlled amounts of regenerant gas evolved from the internal cracking of ammonia, according to the equation:
2NH3xe2x86x92N2+3H2xe2x80x83xe2x80x83(1)
Nitrogen and hydrogen are thus produced, with hydrogen in the greater molar quantity. Hydrogen, a reducing agent, in effect reverses the oxidation of the metal or metal oxide adsorbent during ammonia purification, thereby converting the metal of the adsorbent to a reduced oxidation state for further ammonia purification.
The contaminated ammonia feed stream may be the initial input stream to the process, or it may be a stream which is returned for increased purification after having already been partially purified. In the first case, the feed stream to the purification system will be a stream of fresh (but not decontaminated) fluid NH3, while in the second case the feed stream will usually be the discharge stream from a previous stage of the decontamination process, i.e., ammonia that has been partially decontaminated. The principal contaminants (such as oxygen, water vapor and carbon dioxide) which are removed from the ammonia are adsorbed/chemisorbed from a reaction with reduced metal/metal oxide adsorbent.
Adsorbents are selected from among many known in the art and standardly used in the decontamination of gases, e.g., reduced metals and metal oxides capable of further oxidation, and other catalysts that are stable in the presence of ammonia. Preferred for use in the invention is a reduced iron/manganese oxide that is effective for removal of water, oxygen, carbon dioxide and hydrocarbons.
The preferred system and process of the invention contemplates a plurality of beds having adsorbent for stripping ammonia of contaminants upon pass-through. These beds are interconnected and used on a rotating basis, so that while some beds are being run to decontaminate input ammonia and provide by-product hydrogen from ammonia cracking, others of the beds are being regenerated by passage therethrough of the by-product hydrogen under reducing conditions. By scheduled cycling of the various decontamination beds over run periods usually of several weeks, continual decontamination of ammonia can be obtained without the need for separate input of large quantities of regeneration hydrogen.
Thus, while all of the adsorbent beds may operate simultaneously in the decontamination mode, it is contemplated that not all will, and that those that are not at a given time operating for ammonia decontamination will be undergoing or have just undergone a regeneration of their adsorbent so as to be ready to resume decontamination operation as the adsorbents in the operating beds become spent or otherwise cease to function optimally or properly. Preferably, and to optimize and promote a more continuous purification, stand-by beds (regenerated or new) are activated as necessary while a corresponding bed is targeted for shutdown and regeneration. In this manner, at least one bed in the system is operative at any given time, i.e., in operative communication with an ammonia stream undergoing decontamination.
A small portion of purified ammonia is drawn off from the product stream from the purification bed and directed to a cracking reactor, in which it is decomposed to its component hydrogen and nitrogen gases, which are then directed to a second adsorbent bed, one which has been withdrawn from ammonia purification service for adsorbent regeneration. The hydrogen gas reduces the metal adsorbent to release the accumulated contaminant oxygen, water, hydrocarbons and/or carbon dioxide, which are entrained in the hydrogen gas stream and removed from the regenerating bed and vessel, thus regenerating the adsorbent. The nitrogen generated by ammonia cracking, although essentially inert, may assist in desorption (regeneration) through collisional effect with the contaminants.
Preferably the bed to be regenerated is externally heated, e.g., by a band heating element.
The method permits continuous removal of contaminants from a stream of ammonia to a contaminant content in the purified ammonia stream of not more than 100 ppb, preferably not more than 10-50 ppb, and more preferably not more than about 1 ppb.
The hydrogen gas, as noted, is generated by the decomposition of a portion of the purified ammonia product in a reactor separate from the purification beds/vessels but connected with them for fluid communication of the ammonia, hydrogen and nitrogen. Means for decomposing (cracking) ammonia into its component parts are known in the art, and may be accomplished by a variety of means and apparatuses, preferably those that employ heat in combination with a catalytic material. Conventional alternatives to the use of such thermal catalytic decomposition of ammonia into its constituents include plasma excitation, photoexcitation, electrolytic cracking, RF cracking, and microwave discharge.
In preferred embodiments, a transition metal, mixture of transition metals, or alloy or alloys derived therefrom are used as the cracking catalyst (as distinguished from adsorbent/chemisorbent catalyst). Examples of such materials include Ru, Fe, Mn, Ni, Pt, Pd, Re, Zr, Os, Ir, and Co and their alloys and oxides. The materials may be present in a specialized vessel or bed, e.g., a xe2x80x9creactorxe2x80x9d, in the form of a metal or metallic coating on a high surface area substrate. Preferred for purposes of the invention are Ru, Ni, and/or Re on an alumina substrate coating.
In various embodiments, the portion of ammonia product from one adsorbent bed diverted to decomposition for generation of hydrogen for use in regeneration of a companion bed will be from about 1%-50% by volume of the total ammonia purified that produced in the first bed, more preferably about 2%-30%, and still more preferably about 3%-10%. The regenerating bed is heated to about 200xc2x0-500xc2x0 C., preferably 300-400xc2x0 C., for optimum hydrogen reduction regeneration of the oxidized adsorbent bed. The degree of regeneration obtained, and the conditions used, will depend on such variables as the nature of the metals or metal alloys in the adsorbent and the desired purity level of the ammonia to be obtained. The cracking rate (i.e., portion of diverted ammonia product which is cracked to hydrogen) may be up to 80%. Preferred for catalytic regeneration of the purification adsorbent is a 5%-10% cracking rate. This means that good decontamination of ammonia can be achieved under relatively mild operating conditions, with improved energy usage, reduced energy consumption and loss, and enhanced safety, as compared to high temperature processes of the prior art.
Optionally one can add extra nitrogen (from an outside source) to dilute the diverted portion of ammonia before cracking. This will allow use of less ammonia. The dilution must not, however, be so great that the volume of hydrogen resulting from the cracking is insufficient for regenerate of the second bed. It will be recognized that the limitation on degree of any such dilution will be related to the degree of cracking that occurs in the cracking reactor.
The hydrogen flow regenerates the adsorbent bed by reducing oxidized metal and desorbing the contaminants which accumulated on the catalyst during the bed""s previous decontamination operation. The flow is controlled such that the bed is fully regenerated before, preferably several hours or days before, the operative bed or beds"" decontamination capacity is reached. In this manner, when the operative bed(s) approach capacity for adequate decontamination, the contaminated ammonia fluid stream can within a short period, preferably seconds, be diverted to the regenerated bed and the further decontamination of ammonia proceed substantially without interruption.
Therefore, in one broad aspect, the invention involves a method of decontaminating fluid ammonia comprising contacting contaminated fluid ammonia with an adsorbent to transfer contaminants therein from the contaminated fluid ammonia to the adsorbent, thereby producing decontaminated fluid ammonia, and thereafter regenerating the adsorbent by reacting a portion of the decontaminated fluid ammonia to produce hydrogen gas and contacting the adsorbent with the hydrogen gas to remove transferred contaminants therefrom, the adsorbent thereafter being capable of contact with additional contaminated fluid ammonia for decontamination thereof.
In another embodiment, the invention involves a method as in the preceding paragraph wherein the adsorbent is disposed in a plurality of interconnected vessels, and preferable wherein operation of the method comprises alternatively using each such vessel for decontamination of the contaminated fluid ammonia and regeneration of the adsorbent.
In a further aspect the invention involves a purification system for decontaminating contaminated fluid ammonia comprising adsorbent for reducing the level of contaminants in the fluid ammonia when contacted therewith by absorption of the contaminants from the fluid ammonia onto the adsorbent; and regeneration means for periodic in situ regeneration of the adsorbent by removal of transferred contaminants therefrom by contact with of the adsorbent with the regeneration means, the regeneration means comprising hydrogen gas produced by decomposition of a portion of a fluid ammonia undergoing decontamination.
In yet another embodiment, the invention involves a purification system as in the preceding paragraph for decontaminating fluid ammonia comprising a plurality of vessels each having a body of the adsorbent disposed therein; a first vessel of the plurality having a first body of the adsorbent disposed therein, the first body capable of reducing a level of the contaminants in the fluid ammonia to a desired degree by contact therewith; a second vessel of the plurality having a second body of the adsorbent disposed therein, the second body of the adsorbent having deposited therein a sufficient quantity of the contaminants removed from contaminated ammonia to prevent the second body of the adsorbent from maintaining the desired degree of reduction of level of the contaminants in fluid ammonia by contact therewith; and the regeneration means comprising a reactor in communication with the first vessel and the second vessel for receiving a portion of decontaminated fluid ammonia discharged from the first vessel, decomposing at least a part of the portion of the decontaminated ammonia into hydrogen gas, and passing the hydrogen gas to the second vessel for removing a sufficient amount of the contaminants deposited therein to regenerate the adsorbent in the second vessel for reuse to decontaminate the fluid ammonia.
Other aspects and embodiments will be described below or will be evident from the description below.